Ethernet is a specific implementation of what is generally referred to as a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) protocol. In accordance with such protocols, only a single station is permitted to have access to the medium at any one time. A procedure is specified to resolve conflicts resulting from concurrent access attempts by multiple sources. For Ethernet, the details of this protocol are defined by the IEEE 802.3 specification.
In general, a CSMA/CD protocol operates as follows. Each station that wants to send data over the network, attempts to access the network by transmitting a carrier signal over the network and waits to hear if any other station is also requesting access. Because the signal travels over the network only at a finite speed, the transmitted carrier signal from a first station will take a time T to reach a second station on the network. If the second station attempts to access the network after the first station has asserted its carrier signal on the network but before that signal has reached the second station, the second station will also send its carrier signal over the network. A "collision" will then occur between the two access signals. A collision is simply defined as a condition that results from concurrent transmissions from multiple signal sources. If the second station begins its transmission just prior to the time T, it will become aware of the collision almost immediately. The first station, however, will have to wait another T seconds (i.e., 2T) before it recognizes that a collision has occurred. If a station that is trying to transmit over the network detects a collision, it ceases transmission and waits a random amount of time before again attempting to send its data. Since each station waits a random amount of time before retrying, it is likely that the collision will not occur the next time.
If no other station also wants to send its data at the same time that the first station is trying to send its data, then no collisions will occur and the first station will be allowed to send its data. All other stations will recognize that the network is being used and they will not attempt to access the network until it again becomes quiet.
Typically only a limited number of stations or ports can be supported on a single network segment. However, the IEEE 802.3 specification defines a repeater unit that can be used to expand the number of ports that can operate on the network beyond this limited number. The repeater provides a way of interconnecting multiple segments. By using multiple repeater units the size of the network can be increased significantly. Conventionally, one way others have interconnected multiple repeater units is by way of a synchronous expansion bus (SEB) to which the multiple repeaters are connected. Each repeater unit converts the signal which it receives from a port station (typically a Manchester-encoded signal) into a set of bus signals which conform to some synchronous bus protocol. The bus protocols are typically proprietary to the particular company but they all require expanding the single Manchester-encoded signal to multiple signals requiring from 6 to 10 lines on the SEB. Typically, there will be a clock signal, a data signal, an enable signal and several lines for the collision detection and propagation protocol. Thus, in such systems, each network has its own multi-line bus. Switching a repeater unit from one expanded network to another requires disconnecting the multi-line outputs and inputs of the repeater unit from one SEB and reconnecting them to the other SEB.